One: Alison DiLaurentis
Two: CeCe drake
Three: Bethany young
I think the girl in the grave is CeCe Drake.
CeCe and Bethany are twins. Bethany tried to kill CeCe when they were kids, which is how she ended up in Radley. CeCe never got over this, which is why when Alison told her she was planning on faking her death, CeCe suggested they use her twin sister as the body in her grave (for revenge).
Ali began to visit Bethany. She eventually found a way for her to sneak out of Radley to “come to a party with all of Ali’s friends”.
Bethany saw this as her opportunity to finally get revenge on her sister cece. Since Bethany had grown up with Charles, she knew about his hatred (or obsession?) for Ali, and decided to give him a golden opportunity to finally get his Ali doll, as well.
Ali drugs the girls to cross them off of the “A” list, and prevent them from ruining her fake death.
Bethany and Charles escape and hide outside the dilaurentis house to find Ali. To Bethany’s surprise, she sees her sister CeCe walking into the woods.
Not too long after, Alison walks back from black mailing people into her yard to see her mother talking on the phone through the window. Bethany grabs a rock and hits Ali in the head with it thinking that it’s CeCe. Mrs. D then helps them bury the body.
What they didn’t know was, spencer had been watching the entire time. Freaked out, she ran back to the barn and woke up the other girls telling them all she had witnessed. They peek outside the barn to see what’s going on when they see the girl that spencer has just described as Ali’s killer.
Emily, in an act of rage that this person had hurt Alison, grabbed the first thing she saw (a shovel) and hit her in the head with it.
Unfortunately for Emily, this person was actually CeCe. CeCe had been in the woods blackmailing Melissa Hastings for Ali (there were quite a few people on the list, she needed some help). She was walking back to meet up with Alison for the big night when Em saw her.
The girls freaked (of course) and began arguing. They decided to go back to the barn to figure out what the hell they should do. They agreed to never speak of it again (which wasn’t hard since they’d mixed sleeping pills and alcohol, making them blackout and forget the whole night anyway). They went back out to bury the body to find that it had already been done.
Melissa hadn’t gone far once her conversation with CeCe was over. She was walking back towards Ali’s house when she saw Spencer covered in blood, with a shovel in hand. She walked a little further to find a dead blonde girl she had assumed was Alison. She buried the girl to protect Spencer.
When Emily hit CeCe with the shovel, spencer was trying to stop her, so she was literally right next to her when it happened. This is why Emily remembered hitting someone in hypnosis, and why spencer has flashbacks of blood splattering on her face. Spencer must have picked up the shovel at some point soon after, which is why Melissa thought it was her. (I haven’t figured that part out quite yet.)
I think Ali has been telling the truth about that night. Bethany hit her, mrs. D buried her, and grunwald or maybe Mona fished her out.
Charles believed her to be dead, so he found a new Ali doll, Sara. Charles punished Mona after the lodge fire because he found out that she knew Ali was alive the whole time.
don’t know if this makes sense, but let me know what you think.